Impairment of sarcolemmal permeability after the initiation of acute ischemic myocardial injury was studied using an ionic lanthanum (La3+) probe in electron microscopy. Acute ischemic myocardium was induced by ligation of the left anterior descending coronary artery in dogs. In normal cardiac myocytes La is localized exclusively in the extracellular space. i.e. on the glycocalyx, in the T-system and in intercalated discs, with normal fine structures. In ischemic myocardial cells, La deposits were found in the cytosolic space in 22% of subendocardial cells with mild to moderate, but non-necrotic, fine structural changes as early as 30 min. The number of myocytes with La deposition increased with advancing ischemic injury, and necrotic fine structural changes were recognized following ischemia for 60 min. These results indicate that deposition of La occurs before the appearance of irreversible morphologic alterations in ischemic myocardial cells, and suggest an increased permeability of the sarcolemma for Ca2+ and the development of degradation of plasma membrane integrity.